1. Field of the Invention
The present invention relates to a transmission type or transmission-reflection double type liquid crystal display device which can be reduced in thickness and weight easily and which is excellent in display quality.
The present application is based on Japanese Patent Application No. Hei. 11-361236, which is incorporated herein by reference.
2. Description of the Related Art
Greater reduction in thickness and weight of transmission type liquid-crystal display devices has been demanded for purposes of suppression of increase in weight with the advance of increase in size of television and personal computer display screens, reduction in size and weight of portable personal computers and portable telephone sets, etc. In the meanwhile, it is difficult to reduce thickness and weight of a transmission type liquid-crystal display device provided with a back-lighting system using a background-art bottom type or side-lighting type light pipe. Incidentally, the bottom-type back-lighting system generally has a thickness of not smaller than 4 mm because a light source, a light diffusing plate and a reflector are disposed just under a liquid-crystal display panel. Even the side-lighting type light pipe has a thickness of not smaller than 1 mm under the necessity of light transmission. When a light diffusing plate, a reflector, a prism sheet, etc. are disposed on the side-lighting type light pipe, the total thickness generally reaches a value of not smaller than 3 mm.
An object of the present invention is to develop a transmission type or transmission-reflection double type liquid-crystal display device which can be reduced in thickness and weight easily and which is excellent in display quality.
According to the present invention, there is provided a liquid-crystal display device comprising: a transmission type liquid-crystal panel including a liquid-crystal cell; a light source disposed on at least one side surface of the liquid-crystal display panel; and an optical path changing sheet disposed on a back side opposite to a visual side of the liquid-crystal display panel and provided with optical path changing slopes by which incident light from the light source is reflected toward the visual side of the liquid-crystal display device.
According to the present invention, while incident light from a light source disposed on a side surface of a liquid-crystal display panel by use of liquid-crystal cell substrates is transmitted backward efficiently, the optical path of the transmission light is changed efficiently toward the visual side of the panel through an optical path changing sheet disposed on the back of the panel. Hence, the transmission light can be utilized for liquid-crystal display. A back-lighting system can be formed by the optical path changing sheet excellent in thickness, and formed by arrangement of a light source on its side surface. A transmission type liquid-crystal display device which is excellent in thickness and weight and which is bright and excellent in display quality can be formed. When a reflection layer is disposed on the back surface of the optical path changing sheet, a transmission-reflection double type liquid-crystal display device can be formed easily.
The above description is based on use of a slope reflection type optical path changing sheet. That is, light incident on a side surface or transmission light thereof is reflected by slopes so that the optical path of the light can be changed with good directivity. In a method of scatter reflection by a rough surface as described in JP-A-5-158033, it is difficult to achieve the aforementioned effect. Incidentally, JP-A-5-158033 teaches a reflection type liquid-crystal display device in which illumination light is made incident on a side surface of a liquid-crystal display panel and totally reflected by a visual side cell substrate and in which the reflected light is scattered by a rough surface type reflector so that the scattered light is utilized for display.
In the aforementioned case, however, the light allowed to be utilized for display is the light that emits from the panel because the light comes out of the total reflection condition by scattering. Generally, scattered light exhibits a normal distribution having a direction of positive reflection as a peak. Hence, the aforementioned display light is the light which is hardly utilized efficiently for display greatly inclined with respect to a frontal (vertical) direction. Hence, the display becomes dark in the frontal direction. Nevertheless, intensifying scattering through the rough surface type reflector is unfavorable for display in consideration of viewing in a reflection mode because the quantity of light in the frontal direction is reduced. It is, therefore, necessary to adjust scattering intensity to keep balance between both transmission and reflection modes in such a rough surface type reflection method. It is, however, difficult to obtain scattering intensity favorable to the two modes because the two modes are antinomic to each other.
On the other hand, the slope reflection type optical path changing sheet according to the present invention mainly utilizes light exhibiting a peak in the direction of positive reflection and controls the optical path of the reflected light. Hence, directivity favorable for display, particularly frontal directivity, can be provided easily. Hence, a bright transmission mode can be achieved. Also in a reflection mode, flat portions of the optical path changing sheet except the slopes can be utilized. Hence, the state of light can be balanced easily so as to be favorable to both transmission and reflection modes.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.